1. Field of the Invention
The present invention relates to a 3-stage fluidized bed fine iron ore reducing apparatus, in which a fine iron ore having a wide particle size distribution is reduced to a solid reduced iron within 3-stage fluidized bed furnaces in a stepwise manner before being put into a melting furnace. Particularly, the present invention relates to a 3-stage fluidized bed reducing apparatus in which the gas utilization rate and the reduction rate are improved.
2. Description of the Prior Art
In the conventional blast furnace, iron ore can be reduced based on the fixed bed method, because the solid particles have large sizes. However, in the case where a fine iron ore is to be reduced, if the superficial gas velocity is low as in the case of the fixed bed method, a sticking phenomenon occurs and the operation will finally be interrupted. Therefore, in this case, a fluidized bed method, in which the gas velocity is relatively high so as to make the movements of solid particles brisk, is necessarily employed.
The fluidized bed technology is widely applied to various industrial fields including the gasification of coal, boilers, oil refinery, roasting, the burning of waste materials and the like. Recently this technology has been extensively applied to the melting-reducing method which is a potential iron making technique in near future, and in which a solid iron ore is reduced by using a reducing gas.
In the conventional melting-reducing method, an iron ore is reduced in a cylindrical fluidized bed reducing furnace, and then it is transferred to a melting furnace to make pig iron. In this reducing furnace, the solid iron ore is reduced before melting it. The iron ore which is put into a reducing furnace is reduced within the melter-gasifier by means of a high temperature reducing gas obtained from the burning of fine coal or by means of a natural gas and by making the iron ore react with the reducing gas of high temperature and pressure for a certain period of time. This reducing process is classified into a fixed bed, a moving bed and a fluidized bed depending on the particle size of the iron ore and the mutual contacts between the reducing gas and the solid iron ore. In the case where a fine iron ore is reduced, a solid iron ore is put into a reducing furnace, and a reducing gas is supplied through a gas distributor. Thus the iron ore is fluidized, so that the contact area between the gas and the solid particles can be increased, thereby improving the reactivity. This fluidized bed method is known to be most efficient for the reduction of fine iron ores. So far, the iron ore reducing process which is based on the fluidized method and which is to be commercialized includes DIOS of Japan, and HISMELT and FIOR of Australia.
A fluidized bed reducing furnace is disclosed in Japanese Utility Model Application Laid-open No. Sho-58-217615.
This fluidized bed reducing furnace is illustrated in FIG. 1. Referring to this drawing, the furnace includes a cylindrical reducing furnace 111 and a cyclone 115. The cylindrical reducing furnace 111 includes: a raw iron ore inlet 112, a high temperature reducing gas inlet 113, and a reduced iron ore outlet 114. In addition, a gas distributor 116 is installed in the lower portion of the reducing furnace.
The reducing process in the fluidized bed fine iron ore reducing furnace is carried out in the following manner.
A reducing gas is supplied through the gas distributor 116 at a desired flow rate, and a fine iron ore is put through the inlet 112. Then the iron ore is reacted with the high temperature reducing gas while being agitated. Then after elapsing of some period of time, the reduced fine ore is discharged through the outlet 114.
Under this condition, the pattern of the fluidized bed is as follows. That is, the reducing gas forms gas bubbles within the reducing furnace, and as the gas bubbles pass through the particle layer of the upper portion of the reducing furnace, the gas bubbles grow bigger and bigger.
In the view of economical sense such as the productivity in the fluidized bed reducing furnace, the elutriation of the fine iron ore which flies to the outside of the furnace has to be reduced, the consumption of the reducing gas has to be minimized, and the gas utilization has to be maximized. If these are to be achieved, the particle size of the raw iron ore has to be sternly limited. Therefore, there is a problem that a wide distribution of particle sizes cannot be accommodated.
In the above described conventional fluidized bed type reducing furnace, the particle size distribution of a wide range cannot be allowed, and therefore, it is limited to 1-0.5 mm, -1 mm, or 1-2 mm. However, the particle sizes of the actually available fine iron ore are 8 mm and less. Therefore, they are screened before use, and the large particles are crushed before use. Consequently, the productivity becomes lower and additional process and facilities for screening and crushing are needed, thereby leading to less profitability.
Meanwhile, in order to overcome the above described disadvantages, a twin fluidized bed reducing furnace was disclosed in Korean Patent 74056.
This twin fluidized bed reducing furnace is illustrated in FIG. 1. Referring to FIG. 1, this reducing furnace includes: a first pre-reducing furnace 210 for reducing coarse iron ore; a second pre-reducing furnace 220 for reducing medium and fine iron ores; first and second cyclones 240 and 230; and a hopper 250 for supplying the iron ore.
The first pre-reducing furnace 210 includes: a reducing gas inlet 211 formed on the bottom of it; a gas distributor 212 installed on the lower portion of it; a first outlet 213 formed at a side of lower portion of it; and a second circulating tube 214 formed at a side of upper portion of it, and connected to the second pre-reducing furnace 220. Further, the lower portion of the first pre-reducing furnace 210 is connected through a first circulating tube 231 to the lower portion of the second cyclone 230.
The second pre-reducing furnace 220 includes: a reducing gas supply hole 221 formed on the bottom of it; a gas distributor 222 installed on the lower portion of it; a second outlet 223 formed at a side of lower portion of it; and the upper portion of the furnace 220 being connected to the upper portion of the first cyclone 240.
The upper portion of the first cyclone 240 is connected through a tube to the upper portion of the second cyclone 230. The bottom of the first cyclone 240 is connected through a third circulating tube 241 to a middle portion of the second pre-reducing furnace 220.
The top of the second cyclone 230 is provided with a gas outlet, so that the exhaust gas can be released to the outside after being reacted with the fine iron ore. Meanwhile, the hopper 250 which supplies the iron ore is connected to a side portion of the first circulating tube 231 which connects the first pre-reducing furnace 210 to the second cyclone 230. The first circulating tube 231 and the third circulating tube 241 are respectively provided with a plurality of purging gas supply holes P, so that a clogging would be prevented. A third outlet 242 is formed on an intermediate portion of the third circulating tube 241.
The operating process in the twin fluidized bed iron ore reducing apparatus is carried out in the following manner.
A fine iron ore is supplied from the hopper 250 to the first circulating tube 231. This fine iron ore is transferred to the first pre-reducing furnace 210. Under a controlled gas velocity the coarse iron ore particles form a bubbling or turbulent fluidized bed together with the reducing gas while being reduced. The reduced iron ore is discharged through the outlet 213.
Meanwhile, the medium and fine iron ores are pneumatically transported through the second circulating tube 214 into the lower portion of the second pre-reducing furnace 220 owing to the flow of the high velocity gas which is supplied through the first pre-reducing furnace 210 to the second pre-reducing furnace 220. In this condition, the relatively larger iron ore particles stay within the lower portion of the reaction vessel 220, while the extremely fine particles of 500 .mu.m or less are elutriated into the first cyclone 240, where the captured fine ore is fed back into the second pre-reducing furnace through the second circulating tube 241. Thus the iron ore is reduced for a certain period of time. Further, among the reduced iron ore, the medium iron ore particles are discharged through the second outlet 223, while the fine particles are discharged through the third outlet 242.
A plurality of purging gas supply holes P are formed on the intermediate portions of the second circulating tube 214 and the third circulating tube 241 respectively, and gas supply tubes S are connected to the plurality of the purging gas supply holes P respectively. Thus the circulating tubes 214 and 241 in which the medium/fine iron ores circulated can be prevented from being clogged. Therefore, the particle flows become smooth.
In the fluidized bed reducing furnace of the above mentioned Korean patent, the reduction is carried out by separating the coarse and medium/fine iron ores by proper gas velocities, and therefore, the fluidizing of the iron ore having a wide particle size distribution range can be made stable. Therefore, the concentration of the iron ore can be uniformly maintained, and the reduction degree can be improved.
In the twin fluidized bed, however, the medium/fine iron ores are pneumatically transported through the second circulating tube 214 to the second reducing furnace 220 by a large amount of high velocity gas which has been highly oxidized in the first pre-reducing furnace 210 by the reduction of the coarse iron ore.
The large amount of the oxidized gas is combined with the fresh reducing gas supplied from the gas inlet 221, and therefore, their reducing power becomes lower. Further, the gas flow rate of the second reducing furnace 220 is considerably increased and a large amount of fine iron ore is circulated, and, therefore, the third circulating tube 241 is loaded too much. Consequently, the internal pressure of the reaction vessel severely fluctuates, and fine particles are elutriated out of the system in large amounts. Therefore, the gas consumption amount per ton of the iron ore increases, and the productivity becomes lower. Further, a part of coarse iron ore particles stays in the upper portion of the second reducing furnace 220, with the result, the fluidization of iron ore is adversely affected due to the fluctuation of the internal pressure of the reaction vessel. Further, because the reducing gas which has reacted with the coarse iron ore in the first pre-reducing furnace 210 is emitted through the second pre-reducing furnace and the first cyclone 240 to the outside, this reducing gas cannot be reused, and therefore, the gas utilization rate becomes lower.